Corrosion-resistant weldable martensitic stainless steel, process for the manufacture thereof and articles

ABSTRACT

A corrosion-resistant weldable martensitic steel essentially consists of carbon 0.06 to 0.10 weight percent, chromium 15.1 to 16.5 weight percent, nickel 3.5 to 4.45 weight percent, silicon 0.10 to 0.60 weight percent, manganese 0.20 to 0.50 weight percent, at least one element selected from the group consisting of niobium 0.25 to 0.40 weight percent and zirconium 0.05 to 0.20 weight percent, at least one element selected from the group consisting of yttrium 0.05 to 0.20 weight percent, cerium 0.05 to 0.15 weight percent and lanthanum 0.05 to 0.15 weight percent, phosphorus not exceeding 0.025 weight percent, sulfur not exceeding 0.02 weight percent, copper not exceeding 0.20 weight percent, the remainder being substantially iron and unavoidable nonferrous impurities. The process for the manufacture of the steel comprises the steps of preparing a molten mass of said composition, pouring the molten mass into a mould and permitting it to solidify therein followed by cooling an ingot produced. The cooling step is carried out in at least two stages, the first stage residing in cooling the ingot to a temperature laying in martensite transformation start-end interval but not lower than to 100° C., and then in its immediate heating up to tempering temperatures in the range from 600° to 650° C., whereas each subsequent stage comprises cooling the ingot to martensite transformation temperatures but by at least 50° C. lower than the cooling temperature of the previous stage, thus bringing, with such a multistage cooling, the temperature of the ingot down to a value below the temperature of the end of martensite transformation, followed by final tempering in the temperature range of 600° to 650° C. and subsequent cooling to room temperature.

This is a division of application Ser. No. 91157, filed Nov. 5, 1979,now U.S. Pat. No. 4,299,623.

FIELD OF THE INVENTION

The present invention relates to metallurgy, and more specifically to acorrosion-resistant weldable martensitic steel, process for themanufacture thereof and articles produced of that steel.

The present invention can be used to best advantage in all arts where aneed exists in use of large-sized parts and construction units subjectedto considerable stresses and to corrosive attack at normal and raisedtemperatures as up to 350° C. Also, the present invention can be used tobest advantage in arts where a need exists to use high strengthcorrosion-resistant structural steels which show no deterioration inmechanical and corrosion-resistant properties after being subjected towelding.

BACKGROUND OF THE INVENTION

As a guide to a better understanding of certain features of the presentinvention, it may be noted that at the present time there is a widevariety of corrosion-resistant steels available to the art. Theselection of any particular grade by the user largely depends upon thecombination of characteristics sought, namely, mechanical properties,corrosion cracking resistance, intercrystalline corrosion resistance,workability etc.

Most of practical advantages of these steels depends on both chemicalcomposition of steel and process for the manufacture thereof, and moreparticularly such an important step of the process as thermal treatment.

Steels used in welded constructions of large dimensions subjected toconsiderable stresses and aggressive medium attacks are to meetespecially rigid requirements. Such steels must show a high level ofstrength properties, corrosion cracking resistance and intercrystallinecorrosion resistance. Yet, such steels must possess of good plasticitysufficient to lend themselves to a variety of forming and machiningtechnological operations.

There is well known and widely used a corrosion-resistant martensiticsteel containing in weight percent: carbon, 0.20; manganese, 1.00;silicon, 1.00; phosphorus, 0.040; sulphur, 0.030; chromium, 15.00 to17.00; nickel 1.25 to 2.50; and remainder iron. This steel iscomparatively inexpensive. After hardening from 1050° C. and temper at315° C. this steel has a tensile strength of 140 kgf/mm² and 0.2 percentyield strength of 98 kgf/mm². As a result, this steel is suitable forsuch parts as spindles, gears, racks etc. However, the steels of thetype described show extremely unstable structure. Up to 40 percent ofdelta-ferrite may be contained in the steel structure according to itschemical composition variations. This leads to deterioration offorgeability and ductility, to considerable decrease of transverseimpact strength, and to increase of mechanical anisotropy. Again,stressed parts made of said steel are subjected to intensive corrosioncracking when maintained in aggressive media (for example, hot strongchloride solution).

There is also known a corrosion-resistant austenitic steel containing inweight percent: carbon, 0.08; manganese, 2.00; silicon, 1.00;phosphorus, 0.045; sulphur 0.030; chromium, 17.0 to 19.0; nickel, 9.0 to12.0; titanium to carbon ratio being at least 5 to 1; remainder iron.Said steel shows rather high ductility and good workability. Forgedpieces and bars have in austenitic state (austenitization at 1050° C.) atensile strength of 53 kgf/mm², a 0.2 percent yield strength of 21kgf/mm², an elongation of 40 percent and a reduction in area of 50percent. This grade of steel is costly, however, because of the ratherhigh alloy content, particularly because of large amount of nickel usedtherein. Moreover, said steel has a low strength level and aninclination to corrosion cracking, particularly when chloridesaccumulation takes place.

There is known an austenitic alloy of high nickel content, which alloycontains in weight percent: carbon, 0.10; manganese, 1.50; silicon 1.00;chromium, 19.0 to 23.0; nickel 30.0 to 35.0; titanium, 0.15 to 0.60;remainder iron. This alloy possesses a high corrosion crackingresistance in strong chloride solutions (42 percent MgCl₂ solutionboiling at 154° C., or 0.5 percent NaCl solution boiling at 100° C.,etc.). Its mechanical properties are about the same as thecorrosion-resistant austenitic steel, being a bit less ductile. Tubesmanufactured of said type alloy of high nickel content have inaustenitic state a tensile strength of 49 kgf/mm², a 0.1 percent yieldstrength of 21 kgf/mm², an elongation of 30 percent. But said type alloyis even more expensive than the austenitic steel mentioned hereinabovebecause of significantly higher nickel content.

There is also known a corrosion-resistant ferritic steel containing inweight percent: carbon, 0.08; manganese, 1.00; silicon, 1.00;phosphorus, 0.040; sulphur, 0.030; chromium, 11.5 to 14.5; remainderiron. Said steel is of less cost than the corrosion-resistant austeniticsteel mentioned above. However, said type steel has a low workabilitydue to elevated overheating sensitivity and inclination to heatembrittlement.

Weldability is one of the important characteristics of steel. Theaustenitic steels are easily weldable, but they have disadvantagesdescribed hereinbefore. The alloys of high nickel content are difficultto weld due to cracks appearing at the near-to-weld zone.Overheating-sensitive ferritic steels fail to provide impact strength ofrequired values due to intensive grain growth.

It is generally required on welding the martensitic steels to preheatthe parts to be welded up to temper temperatures ranging from 200° to300° C. in order to keep them free from cold hardening cracksappearance. This results in considerable complexity and expensiveness ofweld process of the martensitic steels.

A group of high-strength corrosion-resistant steels with reversibleadjustable transformation on temper of tempered martensite to austenite(α→γ) have been recently developed. Said steels enjoy a successfulcombination of high tensile strength inherent in martensitic steels andgood ductility, toughness and workability inherent in austenitic steels.One of these steels (cf. "Transaction ASM" 62, No. 4, 1969, pp. 902-914)contains in weight percent: carbon, 0.10; manganese, 0.40 to 0.90;silicon, 0.20 to 0.80; chromium, 11.5 to 13.5; nickel 5.0 to 6.5;molybdenum, 1.2 to 2.0; remainder iron. Said steel provides for forgedpieces having a tensile strength of 85 kgf/mm², a 0.2 percent yieldstrength of 63 kgf/mm², an elongation of 15 to 18 percent, a reductionin area of 50 percent and a Charpy test impact energy level of 11 kgf.m.

To develop such properties of the steel the method of manufacture of thesteel, and particularly the thermal treatment step thereof contributesgreatly.

The process for the manufacture of such a steel resides in preparing amolten mass, pouring the molten mass into a mould and permitting it tosolidify therein followed by cooling an ingot produced.

The ingot or forged piece is then subjected to thermal treatmentcomprising hardening constituting an oil or air cooling, and hightemper, which result in developing up to 30 percent of austenite in thestructure.

However, the field of usefulness of such a seel is rather limited due tolack of austenite stability at low-temperature heatings. Austenitedeveloped in the structure of articles is destabilizated with prolongedheating at a temperature of 300° to 350° C. and upon cooling thearticles to room temperature it is transformated into untemperedmartensite, which results in decrease of the impact strength andinclination of the steel to corrosion cracking.

Moreover, temper at 590°-600° C. used for said steel to develop amaximum of austenite in the structure fails to fully relieve them fromresidual stresses after hardening. The residual stresses of a high levelconstitute a danger of crack formation on cooling ingots, forged piecesand articles of large dimensions fabricated of these steels.

SUMMARY OF THE INVENTION

It is accordingly among the principal objects of the present inventionto provide a corrosion-resistant weldable martensitic steel, a processfor the manufacture thereof and articles produced of this steel, foravoiding the aforesaid drawbacks of the prior art.

Another object of the present invention is to provide acorrosion-resistant weldable martensitic steel and a process for themanufacture thereof, both enabling to ensure high mechanical propertiesof articles produced of this steel.

Still another object of the present invention is to provide acorrosion-resistant weldable martensitic steel and a process for themanufacture thereof enabling to ensure high intercrystalline corrosionresistance and corrosion cracking resistance of articles andconstructions produced of this steel and disposed in water and steam atelevated temperatures and pressures, comprising chlorides and oxygen.

A further object of the present invention is to provide acorrosion-resistant weldable martensitic steel lending itself toconversion into welding wire whose use in welding of articles producedof this steel would ensure the weld joints being equistrength to thebase metal and possessing high ductility, corrosion cracking resistanceand intercrystalline corrosion resistance in aqueous media comprisingchlorides at elevated temperatures.

The present invention will be seen to reside in the combination ofelements, the composition of ingredients forming a corrosion-resistantweldable alloy, and in the process for carrying out the thermaltreatment of intermediate products, all as described herein and thescope of the present invention is set forth in the claims at the end ofthe specification.

With above mentioned and other objects in view, there is proposed acorrosion-resistant weldable martensitic steel comprising carbon,chromium, nickel, silicon, manganese, phosphorus, sulphur and copper,which steel, according to the invention, further comprises at least oneelement selected from the group consisting of niobium 0.25 to 0.40weight percent and zirconium 0.05 to 0.20 weight percent, and at leastone element selected from the group consisting of yttrium 0.05 to 0.20weight percent, cerium 0.05 to 0.15 weight percent and lanthanum 0.05 to0.15 weight percent, the correlation of the other ingredients in percentby weight being as follows:

carbon 0.06 to 0.10;

chromium 15.1 to 16.5;

nickel 3.5 to 4.45;

silicon 0.10 to 0.60;

manganese 0.20 to 0.50;

phosphorus not exceeding 0.025;

sulphur not exceeding 0.02;

copper not exceeding 0.20;

remainder essentially iron and unavoidable nonferrous impurities.

The composition of the steel is viewed as being critical.

Any substantial departure from the ranges set out above results in adisturbance of the structure balance with a resulting sacrifice ofperformance characteristics.

The reasons why the chemical composition of the alloy steel according tothe present invention is limited to the ranges referred to hereinafterwill be explained.

In general, content of unavoidable nonferrous impurities such as tin,antimony and arsenic in the steel according to the present invention isnot regulated. However, when the steel according to the presentinvention is used to produce parts subjected under operating conditionsto neutron irradiation, it is necessary to limit each of the unavoidablenonferrous impurities by content of not more than 0.01 percent, thecorrelation of the ingredients, in percent by weight being as follows:

carbon 0.06 to 0.10;

chromium 15.1 to 16.5;

nickel 3.5 to 4.45;

silicon 0.05 to 0.20;

manganese 0.20 to 0.50;

niobium 0.25 to 0.40;

yttrium 0.05 to 0.10;

lanthanum 0.05 to 0.15;

phosphorus not exceeding 0.20;

sulphur not exceeding 0.015;

copper not exceeding 0.1;

remainder essentially iron.

It is expedient that a niobium to carbon ratio be 4:1 when the steel isused to produce parts and constructions which must possess of extremelyhigh intercrystalline corrosion resistance.

Such a steel enjoys intercrystalline corrosion resistance not only afteran optimum thermal treatment but also after provocative heating.

It is practicable in some instances in the proposed steel to limit acarbon content to the range from 0.06 to 0.07 weight percent and asilicon content to the range from 0.3 to 0.6 weight percent. Such asteel according to the present invention in percent by weight is asfollows:

carbon 0.06 to 0.07

chromium 15.1 to 16.5;

nickel 3.5 to 4.45

silicon 0.3 to 0.6;

manganese 0.20 to 0.50;

niobium 0.25 to 0.40;

zirconium 0.05 to 0.20;

yttrium 0.05 to 0.20;

cerium 0.05 to 0.15;

phosphorus not exceeding 0.025;

sulphur not exceeding 0.20;

copper not exceeding 0.2;

remainder essentially iron and un-a-voidable nonferrous impurities.

A process for the manufacture of a corrosion-resistant weldablemartensitic steel comprises the steps of preparing a molten mass,pouring the molten mass into a mould and permitting it to solidifytherein followed by cooling an ingot produced, wherein, according to theinvention, the step of cooling is carried out in at least two stages,the first stage residing in cooling the ingot to the temperature rangingfrom the temperature of the start of the martensite transformation tothe temperature of the end of the martensite transformation, but notlower than to 100° C., and then in its immediate heating up to temperingtemperatures in the range from 600° to 650° C., whereas each subsequentstage resides in cooling the ingot to the martensite transformationtemperatures but by at least 50° C. lower than the cooling temperatureof the previous stage, thus bringing with such multistage cooling thetemperature of the ingot down to a value below the temperature of theend of martensite transformation, followed by final tempering in thetemperature range from 600° to 650° C. and subsequent cooling to roomtemperature.

Such a process enables to perform multistage controlled phasetransformation of austenite to martensite upon cooling from austenitetemperature range to temperatures of start-end interval of martensitetransformation which is interrupted with heating up to tempertemperatures, thereby providing ingots enjoying a decreased level ofresidual stresses.

This process for the manufacture of a corrosion-resistant weldablemartensitic steel is especially desirable when producing ingots of largedimensions since an ingot of large dimensions can be fractured due toresidual stresses, if not being subjected to thermal treatment accordingto said process.

It is also practicable to use multistage phase transformation ofaustenite to martensite according to the present invention, whenproducing articles of corrosion-resistant weldable martensitic steel,according to the present invention, by means of hot plastic working ofan ingot.

Such method for the manufacture of articles of a corrosion-resistantweldable martensitic steel resides in preparing a molten mass, pouringthe molten mass into a mould, permitting it to solidify therein, hotplastic working of the obtained ingot and its subsequent cooling,wherein, according to the invention, the cooling operation is carriedout in at least two stages, the first stage residing in cooling thearticle to the temperature ranging from the temperature of the start ofmartensite transformation to the temperature of the end of martensitetransformation, but not lower than to 100° C., and then in its immediateheating up to tempering temperatures in the range from 600° to 650° C.,whereas each subsequent stage comprises cooling the article to themartensite transformation temperatures but at least by 50° C. lower thanthe cooling temperature of the previous stage, thus bringing with such amultistage cooling the temperature of the article down to a value belowthe temperature of the end of martensite transformation, followed byfinal tempering in the temperature range from 600° to 650° C. andsubsequent cooling to room temperature.

When practicing the method for the manufacture of articles ofcorrosion-resistant weldable martensitic steel according to the presentinvention the possibility exists of producting a wide variety ofarticles, such as sheet, strip, forged piece and the like.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The present invention will be more understood from the followingdetailed specification taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a histogram group showing mechanical properties of acorrosion-resistant weldable martensitic steel according to the presentinvention from data obtained on testing 132 forged pieces fabricated ofingots of 2,8 tons in weight. The vertical axis shows frequency inpercent of development of certain mechanical properties, the horizontalaxis represents values of these properties. Histogram 1a represents thetensile strength (kgf/mm²), histogram 1b represents the 0.2 percentyield strength (kgf/mm²), histogram 1c represents the elongation (%),histogram 1d represents the reduction in area (%), and histogram 1erepresents the impact strength (semicircular notch) (kgf.m/cm²).

FIG. 2 is a histogram group showing mechanical properties of acorrosion-resistant weldable martensitic steel according to the presentinvention from data obtained on testing 14 forged pieces fabricated ofingots of 12.0 to 13.7 tons in weight. The vertical axis representsfrequency in percent of development of certain mechanical properties,the horizontal axis represents values of these properties. Histogram 2arepresents the tensile strength (kgf/mm²), histogram 2b represents the0.2 percent yield strength (kgf/mm²), histogram 2c represents theelongation (%), histogram 2d represents the reduction in area (%),histogram 2e represents the impact strength (semicircular notch)(kgf.m/cm²), histogram 2f represents the impact strength (V-notch)(kgf.m/cm²).

FIG. 3 is a plot showing the effect of temper temperature and Nb/C rateupon inclination of a corrosion-resistant weldable steel according tothe present invention to intercrystalline corrosion. The horizontal axisrepresents the temper temperature, the left vertical axis represents theniobium to carbon ratio (Nb/C), and the right vertical axis representsthe carbon content in weight percent;

FIG. 4 represents plots showing the effect of temper temperature andvarious media upon the time to failure of specimens under a load. Plot4a shows heats which are outside the present invention and have theniobium to carbon ratio of 0.83 to 1.75, whereas plot 4b shows heatswith the niobium to carbon ratio of 4:1, according to the presentinvention. The vertical axis represents the time to failure of specimensin hours, the horizontal axis represents temper temperature in Celsiusdegrees, Marks , designate distillate at a temperature of 200° to 350°C.; marks , Δ

designate 0.5 percent NaCl solution at a temperature of 100° C.;

marks , designate vapour of 10 percent NaCl boiling solution at atemperature of 200° C.

These marks with arrows designate specimens being uncrushed upon thetesting.

FIG. 5 is a plot showing variation of austenite content in a steeldepending on heating temperature (build-up curves) and tempertemperature (curves with maximum). The vertical axis represents γ-phasecontent in percent, the horizontal axis represents temperature inCelsius degrees.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One steel according to the present invention consists of: carbon 0.06 to0.10 percent, chromium 15.1 to 16.5 percent, nickel 3.5 to 4.5 percent,silicon 0.10 to 0.6 percent, manganese 0.2 to 0.5 percent, niobium 0.25to 0.40 percent, yttrium 0.05 to 0.2 percent, phosphorus not exceeding0.025, sulphur not exceeding 0.02 percent, and copper not more than 0.2percent, remainder being substantially all iron.

Though this steel can comprise at a time all such elements improving itshot plastic workability as yttrium, cerium and lanthanum, it isdesirable to add yttrium only when the steel is used for the manufactureof intermediate products of large dimensions, and that is the case whenyttrium is selected from this group of metals.

From the group of metals consisting of niobium and zirconium, niobium isselected in this case for addition in this steel as a stabilizingelement, thereby providing intercrystalline corrosion resistance of thematerial after thermal treatment.

Another steel according to the present invention essentially consists ofcarbon 0.06 to 0.10 percent, chromium 15.1 to 16.5 percent, silicon 0.10to 0.20 percent, nickel 3.5 to 4.45 percent, manganese 0.2 to 0.5percent. Niobium is added in this steel as a stabilizing element inamount of 0.25 to 0.40 percent which is sufficient to ensureintercrystalline corrosion resistance of the material after optimalthermal treatment hardening at 1050° C. and temper at 635° to 650° C.Though such elements as yttrium, cerium and lanthanum all can be presentin this steel, it is desirable to add yttrium in amount of 0.05 to 0.10percent and lanthanum in amount of 0.05 to 0.15 percent only, to improveability of the steel to hot plastic working, which lanthanum alsoelevates corrosion resistance of the material under neutron irradiation.To rise irradiation embrittlement resistance there are limited strictlyphosphorus, sulphur and nonferrous impurities in this steel. Phosphoruscontent must be not more than 0.02 percent, sulphur not more than 0.015percent, copper not more than 0.1 percent and tin, antimony and arsenic,each, not more than 0.01 percent, remainder iron.

Still another steel according to the present invention which enjoys thebest resistance to intercrystalline corrosion, essentially comprisescarbon in amount of 0.06 to 0.08 percent and niobium in amount at leastfour times as much as carbon, i.e. 0.32 to 0.40 percent, therebyproviding resistance of the steel to intercrystalline corrosion not onlyafter optimal thermal treatment comprising hardening at 1050° C. andtemper at 635° to 650° C., but also after provocative heating at 450° C.The remaining elements are in ranges indicated for the first of theaforesaid steels, i.e.: chromium 15.1 to 16.5 percent, nickel 3.5 to4.45 percent, silicon 0.10 to 0.6 percent, manganese 0.2 to 0.5 percent,phosphorus not exceeding 0.025 percent, sulphur not exceeding 0.02percent and copper not exceeding 0.2 percent and remainder iron. Thoughyttrium and cerium may be both present at a time in this steel, thepresence of yttrium alone in amount of 0.05 to 0.20 percent isdesirable, when this steel is used for the manufacture of intermediateproducts of large dimensions.

Steel according to the present invention is commonly melted in theelectric arc furnaces on clean charge materials.

To improve ductility desirable when producing intermediate products oflarge dimensions, at least one of the rare-earth elements (yttrium,lanthanum, cerium) in amount of 0.05 to 0.15 percent is added either inthe molten bath before the furnace is tapped or in the ladle whentapping. A heat is tapped into a slagged ladle only. For ingots of 1 to15 tons in weight, casting in ingot moulds is carried out via bottomgate. Flow of metal is blasted with argon when casting. Another methodfor producing ingots of steel according to the present invention iselectroslag remelt permits the production of metal free of nonmetallicimpurities. To this end, the metal melted in the electric arc furnacesis casted via bottom gate into slab moulds to produce ingots of 11 to 15tons in weight which then are rolled into slab-electrodes followed byremelting into ingots of 4 to 13 tons in weight by means of electroslagremelt.

Steel produced in the form of ingots is cooled in the ingot moulds,whereas ingots produced by means of electroslag remelt is cooled incrystallizers to the temperature of 100° C., then they are extracted outof the ingot mould or crystallizer and charged into a furnace for thefirst stage of tempering at a temperature of 650° C. The first stage oftempering is followed by cooling of the ingots to 20°-30° C. andsubsequent final tempering thereof at 635° C. (second stage oftempering).

Intermediate products produced of steel according to the presentinvention are as follows: circular, square and octahedral ingots of 1 to15 tons in weight produced by means of open electric arc melting; squareand rectangular ingots of 4 to 13 tons in weight produced by means ofelectroslag remelt; forged pieces up to 15 tons fabricated of openelectric arc melted ingots as well as of electroslag remelt ingots;circular rods of 30 to 180 mm in size, produced of open electric arcmelted ingots as well of electroslag remelt ingots; circular and squareforged pieces of 180 to 400 mm in size fabricated of electroslag remeltingots; slabs of 200 mm in thickness, 800 mm in width and 2000 mm inlength, and welding wire of 1.5 to 5.0 mm in diameter.

Thermal treating of a steel according to the present invention iscarried out as follows. Ingots after being casted, forged pieces atforging temperatures or articles at hardening temperatures are cooled totemperatures laying in martensitic transformation start-end intervalwhich provides a partial transformation of austenite to martensite only,followed by immediate heating up to temper temperatures from 600° to650° C. (first stage of transformation and tempering) then cooling iscarried out to more lower temperatures, however also laying withinstartend interval of martensite transformation followed by heating totempering temperatures (second stage of transformation and tempering)and so on, thereby decreasing cooling temperature with each time till itbecomes equal or somewhat lower than the temperature of the martensitetransformation end; i.e. till to essentially complete transformation ofaustenite to martensite. Then final tempering at temperatures of600°-650° C. follows. Number of stages of transformation and temperrequired is determined from massiveness of ingots, forged pieces andparts and intricacy of their shape (the number should not be less thantwo), whereas cooling temperatures from the start-end interval ofmartensite transformation are chosen such as somewhat equal amount ofmartensite there produced with each cooling stage.

In general, it is practicable, when carrying out thermal treatment ofintricate shape intermediate products of more than 0.5 ton in weightproduced of corrosion-resistant weldable martensitic steel according tothe present invention, to accomplish two-stage transformation and temperof the products with cooling to 100° C. at the first stage andsubsequent tempering at 650° C. followed by second cooling to 20°-30° C.at the second stage and subsequent tempering at 635° C. It is possible,however to carry out thermal treatment comprising more than two stages.

Characteristic examples illustrative of particular aspects of thepresent invention and clearly demonstrating its features and advantagesare given below.

EXAMPLE 1

In Table 1 there are given the chemical ingredients of three steelsaccording to the present invention and of one of the steel outside ofthe invention, with molibdenum present in amount of 1.2 to 2.0 percent.In corrosion-resistant weldable martensitic steels according to thepresent invention molibdenum is present in but residual amounts.

Comparative mechanical properties of the three steels at 20° and 350°C., particularly the tensile strength in kilograms per squaremillimeter, the 0.2 percent yield strength in the same terms, theelongation and reduction in area in percent determined on fivefoldlength specimens, and also impact strength in kilogram-meters per squarecentimeter at room temperature determined on speciments in size of10×10×55 mm with semicircular notch of 2 mm in depth and notch radius of1 mm, and V-notch with angle of 45° the notch radius of 0.25 mm are setout in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Relative                                                                      Heat Chemical composition (weight percent)                                    No.  C  Cr Ni Mn Si Nb Mo Y  Cu S  P  Fe                                      __________________________________________________________________________    1    0.08                                                                             15.4                                                                             4.4                                                                              0.31                                                                             0.25                                                                             0.3                                                                              -- 0.05                                                                             0.06                                                                             0.012                                                                            0.012                                                                            remainder                               2    0.08                                                                             16.2                                                                             3.9                                                                              0.35                                                                             0.45                                                                             0.37                                                                             -- 0.10                                                                             0.08                                                                             0.008                                                                            0.012                                                                            remainder                               3    0.06                                                                             15.5                                                                             4.12                                                                             0.27                                                                             0.32                                                                             0.27                                                                             -- 0.15                                                                             0.10                                                                             0.010                                                                            0.015                                                                            remainder                                4*  0.05                                                                             13.45                                                                            4.35                                                                             0.52                                                                             0.24                                                                             -- 1.56                                                                             -- -- 0.007                                                                            0.015                                                                            remainder                               __________________________________________________________________________     *Outside invention                                                       

                                      TABLE 2                                     __________________________________________________________________________    MECHANICAL PROPERTIES**                                                       at 20° C.                    at 350° C.                                   0.2%                           0.2%                                 Relative                                                                           Tensile                                                                            yield                                                                              Elon-                                                                             Reduc-                                                                            Impact strength                                                                            Tensile                                                                            yield     Reduc-                     Heat strength                                                                           strength                                                                           gation                                                                            tion in                                                                           kgf.m/cm.sup. 2                                                                            strength                                                                           strength                                                                           Elonga-                                                                            tion in                    No.  kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       %   area %                                                                            R = 1 mm                                                                            R = 0.25 mm                                                                          kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       tion %                                                                             area                       __________________________________________________________________________                                                       %                          1    99.3 82.4 19.1                                                                              66.3                                                                              17.0  13.2   83.0 76.2 12.4 63.1                       2    103.0                                                                              83.0 18.4                                                                              65.3                                                                              17.4  14.1   84.1 77.0 12.0 64.2                       3    97.0 79.0 18.2                                                                              54.2                                                                              15.0  11.0   77.4 72.1 13.0 62.4                        4*  85.7 73.7 19.8                                                                              50.5                                                                              17.5  --     71.3 61.1 12.7 45.2                       __________________________________________________________________________     *Outside invention                                                            **Averaged over test data of three specimens                             

The mechanical properties of the three steels according to the presentinvention were determined subsequent to thermal treatment comprising oilhardening at 1050° C. and temper at 650° C., whereas mechanicalproperties of the steel outside the invention were determined subsequentto oil hardening at 990° C. and temper at 600° C.

It should be noted that the steel according to the present invention hasan elevated strength level both at room temperature and a temperature of350° C. as compared to the known steel (cf. heat No. 4), despite to anincreased temper temperature.

For the three heats according to the present invention, the tensilestrength varies within the range from 97 to 103 kilograms per squaremillimeter, the 0.2 percent yield strength is in the range from 79 to 83kilograms per square millimeter as compared to the known steel (cf. heatNo. 4) having the tensile strength of 85 kilograms per square millimeterand 0.2 percent yield strength of 71 kilograms per square millimeter.The remaining values of the mechanical properties are given in Table 2.

Steel according to the present invention is conveniently melted in theelectric arc furnaces. Where desired, ingots of said steel may beproduced by means of electroslag remelt. The furnace is tapped, themetal is teemed into ingot moulds, and the metal is then processed fromingot into forged pieces, slabs and section steel. The metal readilylends itself to hot plastic treatment and works well in the press andmill. The slabs may be converted into sheet, strip down to 40millimeters in thickness or wire down to 5 millimeters in diameter.

As particularly illustrative of the steel according to the presentinvention, especially the mechanical properties of forged piecesfabricated in industry, there is given in Table 3 the chemicalcomposition and in FIG. 1 statistical data of the mechanical properties,namely the tensile strength, the 0.2 percent yield strength, theelongation, the reduction in area, and the impact strength determined onspecimens with semicircular notch-from test data of 132 forged piecesfabricated of ingots of 2.1 to 2.8 tons, and in FIG. 2, statistical dataof the mechanical properties, namely the tensile strength, the 0.2percent yield strength, the elongation, the reduction in area and theimpact strength determined on specimens with semicircular notch andV-notch-from test data of 14 forged pieces fabricated of ingots of 12.0to 13.7 tons in weight. Determination of the mechanical properties ofthe forged pieces are accomplished on longitudinal specimens subsequentto thermal treatment comprising oil hardening at 1050° C. and temper at635°-650° C.

                                      TABLE 3                                     __________________________________________________________________________    Relative                                                                      Heat Chemical composition (weight percent)                                    No.  C  Cr Ni Mn Si Nb Y  Cu S  P  Fe                                         __________________________________________________________________________    5    0.09                                                                             16.14                                                                            4.4                                                                              0.37                                                                             0.25                                                                             0.30                                                                             0.07                                                                             0.06                                                                             0.005                                                                            0.007                                                                            remainder                                  6    0.10                                                                             15.45                                                                            4.23                                                                             0.44                                                                             0.27                                                                             0.33                                                                             0.09                                                                             0.08                                                                             0.005                                                                            0.019                                                                            remainder                                  7    0.09                                                                             15.65                                                                            4.03                                                                             0.37                                                                             0.28                                                                             0.25                                                                             0.08                                                                             0.10                                                                             0.013                                                                            0.013                                                                            remainder                                  8    0.08                                                                             15.75                                                                            4.08                                                                             0.27                                                                             0.27                                                                             0.30                                                                             0.06                                                                             0.05                                                                             0.008                                                                            0.012                                                                            remainder                                  9    0.08                                                                             15.82                                                                            4.16                                                                             0.43                                                                             0.24                                                                             0.37                                                                             0.10                                                                             0.08                                                                             0.009                                                                            0.010                                                                            remainder                                  10   0.06                                                                             15.5                                                                             4.12                                                                             0.27                                                                             0.32                                                                             0.27                                                                             0.08                                                                             0.08                                                                             0.010                                                                            0.015                                                                            remainder                                  11   0.09                                                                             15.74                                                                            4.04                                                                             0.33                                                                             0.30                                                                             0.28                                                                             0.13                                                                             0.08                                                                             0.014                                                                            0.010                                                                            remainder                                  12   0.09                                                                             15.46                                                                            4.30                                                                             0.30                                                                             0.37                                                                             0.25                                                                             0.07                                                                             0.06                                                                             0.010                                                                            0.010                                                                            remainder                                  __________________________________________________________________________     *4 to 14 ingots are produced of each heat                                

In reviewing the data presented above it will be seen that the steelaccording to the present invention produced as forged pieces fabricatedof ingots up to 13.7 tons in weight has a tensile strength of at least92 kilograms per square millimeter, a 0.2 percent yield strength of atleast 75 kilograms per square millimeter, an elongation of at least 11percent, a reduction in area of at least 51 percent, an impact strengthdetermined on specimens with semicircular notch of at least 12kilogram-meters per square centimeter and that determined on specimenswith V-notch of at least 7 kilogram-meters per square centimeter.

Likewise, in other examples illustrative of the steel according to thepresent invention, the material with more restricted contents of carbon,niobium, silicon and also with additions of rareearth elements(lanthanum, cerium) and transition group metals (zirconium) enjoys themechanical properties of the same level as described in this example.

EXAMPLE 2

The chemical composition of three corrosion-resistant weldablemartensitic steels, in which, as compared to the steel presented in theExample 1, silicon content is limited to 0.20 percent maximum lanthanumin amount of 0.05 to 0.15 percent is added, and also contents ofnonferrous impurities, sulphur and phosphorus, are limited,particularly, copper to 0.1 percent maximum, each of tin, antimony andarsenic to 0.01 percent maximum, sulphur to 0.015 percent maximum,phosphorus to 0.02 percent maximum is given in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Relative                                                                      Heat Chemical composition (weight percent)                                    No.  C  Si Mn Cr Ni P  S  Nb Y  La Cu Sn Sb As Fe                             __________________________________________________________________________    13   0.06                                                                             0.05                                                                             0.30                                                                             15.10                                                                            3.5                                                                              0.006                                                                            0.005                                                                            0.25                                                                             0.05                                                                             0.05                                                                             0.01                                                                             0.002                                                                            0.001                                                                            0.003                                                                            remainder                      14   0.08                                                                             0.12                                                                             0.43                                                                             16.05                                                                            4.03                                                                             0.010                                                                            0.011                                                                            0.33                                                                             0.07                                                                             0.10                                                                             0.08                                                                             0.005                                                                            0.005                                                                            0.007                                                                            remainder                      15   0.10                                                                             0.20                                                                             0.5                                                                              16.5                                                                             4.4                                                                              0.010                                                                            0.015                                                                            0.40                                                                             0.10                                                                             0.15                                                                             0.10                                                                             0.010                                                                            0.010                                                                            0.010                                                                            remainder                      __________________________________________________________________________

Comparative mechanical properties of these three steels in startingthermal treatment condition (oil hardening at 1050° C. and tempering at650° C.) and subsequent to irradiation by flux of 1.4.10²⁰ neutrons persquare centimeter, particularly the tensile strength in kilograms persquare millimeter, the 0.2 percent yield strength in the same terms, theelongation in percent determined on fivefold length specimens, theimpact strength determined on specimens of 10×10×55 mm in size withV-notch of 45° and notch radius of 0.25 mm in kilogram-meters per squarecentimeter, and also brittleness critical temperature values in Celsiusdegrees prior and subsequent to the irradiation, and irradiationembrittlement ratio values are given in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                   Mechanical properties at 20° C.                                                             Brittle-                                                                           Brittle-                                                                           Irradia-                             Irradiation    0.2%            ment cri-                                                                          ment cri-                                                                          tion em-                        Relative                                                                           conditions                                                                              Tensile                                                                            yield                                                                              Elon-                                                                             Impact tical tem-                                                                         tical tem-                                                                         brittle-                        heat Tempera-                                                                           Flux neu-                                                                          strength                                                                           tensile                                                                            gation                                                                            strength                                                                             perature                                                                           perature                                                                           ment ra-                        No.  ture °C.                                                                    tron/cm.sup.2                                                                      kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       %   kgf · m/cm.sup.2                                                            °C.                                                                         shift °C.                                                                   tio**                           __________________________________________________________________________    13   --   --   94.4 78.0 18.0                                                                              20.0    -100                                                                              --   --                                   270-350                                                                            1.4 · 10.sup.20                                                           108.0                                                                              89.0 16.0                                                                              17.5   -60  40   7.7                             14   --   --   96.0 83.0 15.5                                                                              16.5   -60  --   --                                   270-350                                                                            1.4 · 10.sup.20                                                           104.5                                                                              95.0 14.0                                                                              15.0   -40  20   3.8                             15   --   --   102.5                                                                              89.4 13.3                                                                              14.0   -90  --   --                                   270-350                                                                            1.4 · 10.sup.20                                                           110.0                                                                              98.0 12.5                                                                              11.0   -50  40   7.7                             __________________________________________________________________________     *Brittleness critical temperature was determined from impact strength         having average amounts of 6.0 kilogrammeters per square centimeter with       allowance for minimum value of 4.2 kilogrammeters per square centimeter.      ##STR1##                                                                 

It is noted from the information presented in Table 5 that themechanical properties of the steel according to the present invention instarting thermal treatment condition are at a level of the properties ofthe steel according to Table 2. Subsequent to irradiation with flux of1.4.10²⁰ neutron/cm² at 270°-350° C. some strengthening developsconcurrently with modest decrease of the elongation and impact strengthof the steel. As this takes place, the tensile strength rises from94.4-102.5 to 104.5-110.0 kilograms per square millimeter, the 0.2percent yield strength rises from 78.0-89.4 to 89-98 kilograms persquare millimeter, the elongation drops from 13.3-18.0 to 12.5-16.0percent, the impact strength drops from 14.0-20.1 to 11.0-17.5kilogram-meters per square centimeter and the brittlement criticaltemperature rises from (-60)-(-100) to (-40)-(-60)Celsius degrees.

EXAMPLE 3

The chemical composition of three heats of corrosion-resistant weldablemartensitic steel which lends itself to use as a welding wire, accordingto the present invention, is given in table 6.

Compared to the steel of Example 1 it has somewhat restricted carboncontent in amount of 0.06 to 0.07 percent, silicon in amount of 0.3 to0.6 and zirconium and cerium are added in amounts of 0.05 to 0.20percent and 0.05 to 0.15 percent, respectively.

Blanks of steel according to the present invention of 150 mm inthickness are welded by means of argon-arc welding. The mechanicalproperties at 20 and 350° C. of the base metal and the weld jointproduced with use of the welding wire according to the presentinvention, namely the tensile strength in kilograms per squaremillimeter, the 0.2 percent yield strength in the same terms, theelongation and reduction in area in percent determined on specimens offivefold length, and also the impact strength at room temperaturedetermined on notched specimens of 10×10×55 mm in size with semicircularnotch of 2 mm in depth and 1 mm in notch radius in kilogram-meters persquare centimeter, are given in Table 7.

It is noted from the information presented in Table 7 that the weldjoint accomplished with use of the welding wire according to the presentinvention is practically equivalent to the base metal, particularly atroom temperature tests, and has the ductility and impact strength of thesame values as the base metal. For instance, the tensile strength of theweld joint at room temperature is 96.1 kilograms per square millimeter,compared to 99.2 kilograms per square millimeter of the base metal.Values of elongation, reduction in area and impact strength of the weldjoint are close to those of the base metal in ranges of 18.6 to 18.8percent, 70.0 to 70.9 percent and 20.3 to 21.9 kilogram-meters persquare centimeter, respectively. When testing at 350° C. the differencein the strength properties of the weld joint and those of base metal issomewhat more considerable. For instance, a tensile strength of the weldjoint at 350° C. is 79.2 kilograms per square millimeter as compared to86 kilograms per square millimeter of the base metal. Nevertheless,absolute values of the strength properties of the weld joint are highenough and ensure practically equistrength to the base metal.

Melting methods for said steel are the same as for that described in theExample 1 and comprise melting of the steel in the electric arcfurnaces. After the furnace is tapped the melted metal is teemed intoingot moulds and the ingot is then fabricated into a sheet bar which isrolled in the hot mill into a wire of 6 mm in diameter followed bystretching in the wire mill into a wire of 1.5 to 5.0 mm.

                                      TABLE 6                                     __________________________________________________________________________    Relative                                                                      heat      Chemical composition                                                Material                                                                           No.  C  Cr Ni                                                                              Mn Si Cu Nb Zr S  P  Y  Ce Fe                               __________________________________________________________________________    Welding                                                                       wire 16   0.06                                                                             16.4                                                                             4.4                                                                             0.46                                                                             0.60                                                                             0.05                                                                             0.40                                                                             0.20                                                                             0.007                                                                            0.010                                                                            0.20                                                                             0.15                                                                             remainder                        Base                                                                          metal                                                                              17   0.09                                                                             15.4                                                                             4.3                                                                             0.31                                                                             0.39                                                                             0.06                                                                             0.37                                                                             -- 0.008                                                                            0.013                                                                            0.07                                                                             -- remainder                        Weld                                                                          metal                                                                              18   0.06                                                                             16.2                                                                             4.4                                                                             0.49                                                                             0.58                                                                             0.06                                                                             0.38                                                                             0.18                                                                             0.007                                                                            0.011                                                                            0.15                                                                             0.12                                                                             remainder                        __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________             Mechanical properties                                                         at 20° C.          at 350° C.                               Rela-    0.2%                      0.2%                                       tive                                                                              Tensile                                                                            yield     Reduc-                                                                            Impact Tensile                                                                            yield                                      heat                                                                              strength                                                                           strength                                                                           Elonga-                                                                            tion in                                                                           strength                                                                             strength                                                                           strength                                                                           Elonga-                                                                            Reduction                   Material                                                                           No. kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       tion %                                                                             area %                                                                            kgf · m/cm.sup.2                                                            kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       tion %                                                                             in area                     __________________________________________________________________________                                                      %                           Weld 18  96.1 --   18.6 70.0                                                                              21.9   79.2 --   15.8 67.3                        joint                                                                         Base 17  99.2 83.7 18.8 70.9                                                                              20.3   86.0 82.0 13.1 67.8                        metal                                                                         __________________________________________________________________________     Note:                                                                         1. Test data are averaged over three specimens.                               2. Specimens were tested subsequent to thermal treatment comprising oil       hardening at 1050° C. and temper at 650° C.                     3. Specimens were cut out transversely to the weld. Impact strength test      notch in the specimens is accomplished in weld metal.                    

EXAMPLE 4

In table 8 the chemical composition of steel heats according to thepresent invention is given, in which, as compared to the steel ofExample 1, carbon content is restricted to amount of 0.06 to 0.08percent and niobium content is restricted to amount of 0.32 to 0.40percent which ensures the niobium to carbon ratio of at least 4:1. Tocompare with there are some steels also melted, which steels have carbonand niobium contents in the ranges of the Example 1, namely carbon of0.06 to 0.10 percent and niobium of 0.25 to 0.40 percent. A group ofheats also melted, which heats have carbon and niobium contents outsideof the present invention. In all of these heats the contents of theessential alloys, i.e. chromium and nickel, were somewhat equal andranged from 15.0 to 16.5 and 3.73 to 4.35 percentrespectively, whereasthe niobium to carbon ratio varied from 0.8 to 4.6.

Inclination of the steel to intercrystalline corrosion and corrosioncracking were determined from said heats.

Inclination to intercrystalline corrosion was determined subsequent toexposure for 24 hours in boiling solution of blue vitriol and sulphuricacid with presence of copper chip. Intercrystalline ruptures (asrelative to initial austenite grain) were identified with appearance ofcracks upon bending of specimens through 90 deg and by means ofmetallographic analyses. Flat specimens in size of 2×6×80, 2×20×90,2×10×70 mm with a constant given deformation in pure bending zone weresubjected to corrosion cracking test. The specimens upon initial givenstresses amounting to 0.8 to 0.2 percent yield strength were subjectedto exposure in distillate with chlorides and oxygen contents of 0.05mg/kg each with or no addition (12 g/kg) of boric acid at temperaturesof 200° to 350° C., in 0.5 percent NaCl solution at 100° C., and invapor of boiling 10 percent NaCl solution at 200° C. Corrosion crackingresistance was assessed with time duration up to the moment of a firstcrack appearance visible at 16X magnification.

                  TABLE 8                                                         ______________________________________                                        Relative Chemical composition                                                 heat     (weight percent)      Nb/C                                           No.      C         Cr     Ni      Nb   ratio                                  ______________________________________                                        I-1*     0.10      15.10  4.26    0.08 0.80                                   I-2*     0.12      15.88  4.34    0.10 0.83                                   I-3*     0.11      15.00  4.13    0.10 0.91                                   I-4*     0.11      15.60  4.30    0.11 1.00                                   II-1*    0.07      15.57  4.20    0.09 1.28                                   II-2*    0.09      16.45  4.23    0.13 1.45                                   II-3*    0.08      16.44  4.25    0.14 1.75                                   II-4*    0.09      15.46  4.35    0.20 2.20                                   II-5*    0.05      16.25  3.94    0.12 2.40                                   II-6*    0.06      14.70  3.30    1.17 2.84                                   II-7*    0.06      16.20  4.11    0.18 3.00                                   III-1    0.09      15.74  4.04    0.28 3.10                                   III-2    0.09      15.73  3.73    0.33 3.70                                   III-3    0.08      15.75  4.08    0.30 3.80                                   III-4    0.08      16.50  3.90    0.32 4.00                                   III-5    0.06      15.50  4.12    0.27 4.50                                   III-6    0.08      15.38  4.26    0.37 4.60                                   ______________________________________                                         *Outside invention                                                       

Data illustrative of temper temperature and the Nb/C ratio effect uponinclination of heats investigated to intercrystalline corrosion (eachpoint represents test data from 3-4 specimens), are given in FIG. 3. Itis to be seen from FIG. 3 that with a carbon content of more than 0.1percent and a niobium content of less than 0.1 percent, i.e. with theniobium to carbon ratio of less than 1.0, the steel inclines tointercrystalline corrosion in the temperature range of 300° to 700° C.independent of temper temperature (oil hardening at 1050° C.). With acarbon content of less than 0.1 percent and an elevated niobium contentup to 0.18 percent the temperature range where the steel inclines tointercrystalline corrosion is restricted. In this case, the heats with acarbon content of 0.05 to -0.09 percent and a niobium content of 0.09 to0.18 percent, i.e. with the niobium to carbon ratio of 1.0 to 3.0, allincline after temper at provocative temperature of 450° C. tointercrystalline corrosion. With a niobium content of more than 0.25percent and a carbon content of less than 0.1 percent. i.e. with theniobium to carbon ratio of more than 3.0 steel according to the presentinvention shows no more inclination to intercrystalline corrosion. Forinstance, a steel with a niobium content of 0.32 percent and a carboncontent of 0.08 percent, that is, with the niobium to carbon ratio of4.0, shows no inclination to intercrystalline corrosion not onlysubsequent to optimum thermal treatment comprising oil hardening at1050° C. and temper at 650° C., but also after provocative heating at450° C.

To suppress inclination to intercrystalline corrosion the niobium tocarbon ratio must amount of not less than 10.0 for the type 18-8austenitic steels, whereas it must be of not less than 28 for the type20-45 alloys of high nickel content. A decreased niobium to carbon ratioof not less than 3.0 required to stabilize a corrosion-resistant steelaccording to the present invention, as compared to that of austeniticsteel, is attributed to a more disperse structure of martensite, a greatdeal of phase boundaries, and a less solubility of carbideformingelements in ferrite as compared to austenite.

It is to be seen from FIG. 4 that endurance of steel specimens with theniobium to carbon ratio equal to 4.0 at any temper temperature is higherthan that of a steel with the niobium to carbon ratio ranging from 0.83to 1.75. Subsequent to temper at 300° C. corrosion cracking tests showconsiderable variability of results. The least resistance is observedsubsequent to temper at 450° C. The temper temperature increase up to600° C. does not overcome inclination of a steel with the niobium tocarbon ratio of 0.83 to 1.75 to corrosion crackng and it is not untilsubsequent to temper at 650° C. that a marked improvement of theresistance is observed. As for steels with the niobium to carbon ratioof 4.0, their resistance rises markedly even subsequent to temper at600° C.

Melting processes for said steel are the same as for the steel describedin the Example 1 and comprise melting of the steel in the electric arcfurnaces as well as producing of ingots by means of electroslag remelt.After the melting is over, the melted metal is teemed into ingot mouldsor remelted in crystallizers. The ingots are then converted into forgedpieces, slabs, and shape stock.

EXAMPLE 5

As illustrative of properties of the steel according to the presentinvention and effect of the chemical composition of this steel upon themechanical properties and the impact strength under conditions ofprolonged use at elevated temperatures, four heats of chromium-nickelmartensitic steels of closely related chemical compositions are given inTable 9. Three of these heats answer to the requirements of the presentinvention, while the remaining one of rather similar composition differstherefrom.

In Table 9 there are presented compositions of steels comprisingchromium of 13.45 to 15.78 percent and nickel of 3.9 to 5.35 percent.The heat, which is outside the present invention, further comprisesmolybdenum in amount of 1.56 percent. The mechanical properties of thesesteels and the heat embrittlement resistance thereof following prolongedexposure for 500-10000 hours at a temperature of 340° C. are set forthin Table 10. This Table presents the mechanical properties and impactstrength of steels following thermal treatment comprising oil or waterhardening at 990°-1050° C. and temper at 600°-650° C. for 2-12 hours,both prior to exposure to elevated temperature and following exposurefor 500-10000 hours at a temperature 340° C. In the table the tensilestrength, the 0.2 percent yield strength, the elongation, and thereduction in area, all determined on fivefold length specimens, and theimpact strength determined on notched specimens in size of 10×10×55 mmwith semicircular notch of 2 mm in depth and notch radius of 1 mm andV-notch with notch radius of 0.25 mm are presented.

It is to be seen from Table 10 that ductility of the steel, namely theelongation and the reduction in area varies but slightly followedprolonged exposures at 340° C., whereas strength level somewhat rises.Thus, the tensile strength of the heats tested increases from 85.7-102.3kilograms per square millimeter (in initial conditions) to 91.3-115.0kilograms per square millimeter (subsequent to exposure for 500-10000hours at 340° C.), whereas the 0.2 percent yield strength rises from73.7-83.7 kilograms per square millimeter (in initial conditions) to77.7-105.2 kilograms per square millimeter (subsequent to exposure for500-10000 hours at 340° C.). The impact strength of the steel somewhatdrops following prolonged heat exposures. Impact strength valuesfollowing exposure for 500-10000 hours at 340° C. drop from 12.2-20.5kilogram-meters per square centimeter to 10.6-16.4 kilogram-meters persquare centimeter for the specimens with notch radius of 1.0 mm and from8.1-16.4 kilogram-meters per square centimeter to 7.2-13.5kilogram-meters per square centimeter for the specimens with notchradius of 0.25 mm (in initial conditions).

Roentgenographic analysis of the specimens on a diffractometer with FeKαirradiation shows that austenite content in the structure falls afterprolonged exposures at 340° C. It falls from 16 percent (in initialconditions) to 8-10 percent for the heat 21 steel, which evidencesdestabilization of austenite (upon additional heatings) a part of whichis transformed into untempered martensite following cooling below themartensite transformation start temperature. Additional prolongedexposures at 340° C. also result in decomposition of solid solution.Physicochemical and X-ray analysis in CuKα irradiation show thatcarbides of cement type (Fe, Cr)₃ C fall out in the steel structureaccording to the present invention upon heating at 340° C. Thus,decomposition of the supersaturated solid solution upon prolongedheating at 340° C. and appearance of untempered martensite transformedupon cooling from austenite destabilized upon additional heatings resultin aforementioned decrease of the impact strength of the steel.

                                      TABLE 9                                     __________________________________________________________________________    Relative                                                                      heat Chemical composition (weight percent)                                    No.  C  Cr Ni Mn Si Nb  Cu Y  S  P  Fe                                        __________________________________________________________________________    19*  0.09                                                                             15.38                                                                            4.26                                                                             0.31                                                                             0.39                                                                             0.37                                                                              0.06                                                                             0.07                                                                             0.008                                                                            0.013                                                                            Remainder                                 20*  0.08                                                                             15.75                                                                            4.08                                                                             0.27                                                                             0.27                                                                             0.30                                                                              0.05                                                                             0.10                                                                             0.008                                                                            0.012                                                                            Remainder                                 21** 0.08                                                                             15.78                                                                            3.90                                                                             0.27                                                                             0.35                                                                             0.25                                                                              0.07                                                                             0.05                                                                             0.008                                                                            0.024                                                                            Remainder                                                     Mo  Al                                                     *** 0.05                                                                             13.45                                                                            5.35                                                                             0.52                                                                             0.24                                                                             1.56                                                                              0.17                                                                             -- 0.007                                                                            0.015                                                                            Remainder                                 __________________________________________________________________________     *Steels according to the invention                                            **Steel according to the invention enjoying a best combination of             properties following prolonged heatings.                                      ***The type 2RMO steel, outside invention.                               

                                      TABLE 10                                    __________________________________________________________________________                       Mechanical properties at 20° C.                                             0.2%             Impact strength                      Prolonged heating  Tensile                                                                            yield      Reduction                                                                           kgf, m/cm.sup.2                      Temperature                                                                            Duration                                                                           Relative                                                                           strength                                                                           strength                                                                           Elongation                                                                          in area                                                                             Radius =                                                                           Radius =                        °C.                                                                             hours                                                                              heat No.                                                                           kgf/mm.sup.2                                                                       kgf/mm.sup.2                                                                       %     %     1.0 mm                                                                             0.25 mm                         1        2    3    4    5    6     7     8    9                               __________________________________________________________________________    Initial condi-                                                                tions: oil hard-                                                              ening at 1050° C.                                                      + temper at   19   99.2 83.7 18.8  70.9  20.3 16.4                            650° C., 2 hours,                                                      air                                                                           Initial condi-                                                                tions: oil harden-                                                            ing at 1050° C. +                                                      temper at 580° C.                                                                    20   95.0 74.0 19.0  58.0  12.2 8.1                             6 hours + 645° C.,                                                     12 hours                                                                      Initial condi-                                                                tions: oil harden-                                                            ing at 1050° C. +                                                                    21   102.3                                                                              81.5 19.2  67.3  20.5 15.0                            temper at 650° C.,                                                     2 hours, air                                                                  Initial conditions:                                                           water harden-                                                                 ing at 990° C.                                                                            85.7 73.7 19.8  50.5  17.5 --                              + temper at                                                                   620° C., 6                                                             hours                                                                                   500 21   105.8                                                                              101.2                                                                              17.6  64.3  15.7 13.5                                          19   103.8                                                                              95.8 17.9  65.5  16.4 10.4                                     1000 20   93.3 77.7 21.2  64.0  12.6 8.0                                           21   106.4                                                                              100.2                                                                              20.1  63.4  13.2 10.5                                               91.3 82.4 17.8  54.3  11.4 --                                            19   103.7                                                                              95.5 20.1  66.4  --   --                              340      3000 21   108.6                                                                              103.3                                                                              19.0  62.3  14.8 13.5                                               93.1 81.2 17.3  52.2  10.9 --                                            20   97.7 83.3 20.8  64.0  11.8 7.2                                      5000 21   115.0                                                                              105.2                                                                              18.0  58.5  13.8 12.3                                               95.6 85.6 19.2  54.2  10.6 --                                       10000                                                                              20   93.8 83.0 20.2  62.0  10.9 --                                            21   --   --   --    --    13.2 --                              __________________________________________________________________________     *Averaged over test data of three specimens                              

The type 2RMO steel which is outside the present invention shows themost decrease of impact strength values. It is because the type 2RMOsteel comprises up to 25-30 percent of austenite (FIG. 5) subsequent tooptimum thermal treatment consisting of hardening at 990° C. and temperat 620° C. However, such austenite is relatively unstable and much of itis transformed into untempered martensite upon additional prolongedheatings which leads to marked decrease of impact strength values.Austenite content in the steel according to the present inventionsubsequent to optimum thermal treatment consisting of hardening at 1050°C. and temper at 650° C. is decreased to amount about 10-15 percent. Inthis case, the degree of decomposition of austenite upon additionalprolonged heatings is lowered. Related to this is a diminished fall ofimpact strength values upon prolonged heat exposures, and hence, anincreased heat embrittlement resistance. Of steels according to thepresent invention, the steel of heat 21 having the least nickel content(3.9 percent) and the somewhat lower niobium content (0.25 percent) withthe carbon content of 0.08 percent shows the most improved heatembrittlement resistance. Such a content of said elements is consideredas being preferred for parts operating at elevated temperatures for along time.

EXAMPLE 6

A steel comprising carbon 0.09 percent, manganese 0.33 percent, silicon0.38 percent, chromium 15.5 percent, nickel 3.86 percent, niobium 0.3percent, yttrium 0.07 percent, copper 0.12 percent, phosphorus 0.012percent, remainder iron, is melted in the open electric arc furnace,teemed into ingot moulds of 6.5 and 14.0 tons in capacity, in whichcrystallization and cooling of ingots to 400° C. (ingot of 14 tons) andto 300° C. (ingot of 6.5 tons) take place. The ingots of 14 and 6.5 tonsin weight are then extracted out of the ingot moulds and cooled in airto 100° and 80° C. relatively. After this takes place (not longer thanin two hours) the ingots are charged in a furnace heated up to 300° C.(exposure at the charge temperature for two hours) and the temperatureof the furnace is then raised to 650° C. at the rate of 50 Celsiusdegree per hour (first stage of tempering). The exposure duration at650° C. depends on the charge mass, particularly the exposure lasts for20 hours for a charge of 40 tons in mass. The 20-hour exposure at 650°C. is followed by cooling of the ingots in the furnace to 300° C. andsubsequent air cooling to room temperature. A second stage of temperingbegins in two hours at latest subsequent to cooling of the ingots. Thesecond stage comprises charging of the furnace at a temperature nothigher than 300° C., exposure at the charge temperature for not lessthan two hours, heating up to 630° C. at the rate of 50° C. per hour,exposure at 630° C. for 20 hours, cooling within the furnace to 300° C.and then air cooling. Hardness developed in the ingots subsequent to thetwo-stage tempering is 272-287 HB.

EXAMPLE 7

Six steel heats whose chemical composition is set forth in Table 11 aremelted in the open electric arc furnace. The steel is teemed into ingotmoulds in capacity of 2.7-2.8, 12.0, 13.0 and 13.7 tons wherecrystallization of the steel takes place. Subsequent to cooling of theingots to 100° C. the two-stage tempering is accomplished according tothe conditions described in the previous example. The annealed ingots inweight of 2.7-2.8, 12.0, 13.0 and 13.7 tons are the forged at thetemperature range of 1200°-950° C. followed by collecting of the forgedpieces in the furnace at temperatures of 600°-640° C. Subsequent to thecollecting of the forged pieces in the furnace their cooling within thefurnace is accomplished at the rate in the average of 16° C. per hour to300° C. followed by air cooling to 100° C. In an hour the forged piecesare charged in the furnace heated up to a temperature of 250°-300° C.followed by rising of the temperature of the furnace up to 650° C. atthe rate of 50° C. per hour (first stage of annealing). Exposureduration at 650° C. depends on the charge mass, particularly theexposure lasts for 38 hours for a charge of 75 tons in mass. The 38-hourexposure at 650° C. is followed by cooling of the forged pieces withinthe furnace to 270° C. and subsequent air cooling to room temperature.In at latest two hours subsequent to cooling of the forged pieces toroom temperature they are charged into the furnace for a second stage ofannealing.

The second stage of annealing comprises furnace charge at 300° C.,exposure for three hours at the charge temperature, heating up to 630°C. at the rate of 50° C. per hour, exposure for 46 hours at 630° C.,cooling within the furnace to 350° C. followed by air cooling. Hardnessdeveloped in the ingots subsequent to the two-stage annealing is 255-286HB.

EXAMPLE 8

Three steel heats whose chemical composition is set forth in Table 11(relative heat Nos. 23, 24, 25) are melted in the open electric arcfurnace. The steel is teemed into ingot moulds in capacity of 12.0, 13.0and 13.7 tons where crystallization of the steel takes place. Subsequentto cooling of the ingots to 100° C. a two-stage tempering isaccomplished according to the conditions described in Example 7, thetempered ingots in weight of 12.0, 13.0 and 13.7 tons are then forged atthe temperatures range of 1200°-950° C., followed by tempering of theforged pieces according to the conditions described in Example 7.

                                      TABLE 11                                    __________________________________________________________________________    Relative                                                                      heat Chemical composition (weight percent)                                    No.  C  Mn Si Cr Ni Nb Y  Cu S  P  Fe                                         __________________________________________________________________________    22   0.08                                                                             0.27                                                                             0.27                                                                             15.75                                                                            4.08                                                                             0.30                                                                             0.08                                                                             0.05                                                                             0.008                                                                            0.012                                                                            Remainder                                  23   0.09                                                                             0.38                                                                             0.36                                                                             15.73                                                                            3.73                                                                             0.33                                                                             0.07                                                                             0.10                                                                             0.011                                                                            0.010                                                                            Remainder                                  24   0.08                                                                             0.43                                                                             0.24                                                                             15.82                                                                            4.16                                                                             0.37                                                                             0.09                                                                             0.08                                                                             0.009                                                                            0.010                                                                            Remainder                                  25   0.06                                                                             0.27                                                                             0.32                                                                             15.50                                                                            4.12                                                                             0.27                                                                             0.06                                                                             0.08                                                                             0.010                                                                            0.015                                                                            Remainder                                  26   0.09                                                                             0.33                                                                             0.30                                                                             15.74                                                                            4.04                                                                             0.28                                                                             0.11                                                                             0.08                                                                             0.014                                                                            0.010                                                                            Remainder                                  27   0.08                                                                             0.28                                                                             0.32                                                                             16.05                                                                            4.25                                                                             0.27                                                                             0.13                                                                             0.08                                                                             0.010                                                                            0.014                                                                            Remainder                                  __________________________________________________________________________

The tempered forged pieces are subjected to mechanical treatment, sharpangles, sites of surface intersections and edges are therewith rounded.After this takes place, thermal treatment of the forged pieces isaccomplished, which forged pieces up to 150 mm in section are subjectedto two-stage temper. Said thermal conditions are used for treatment offorged pieces of 510 mm in major diameter, 200 mm in minor diameter and4000 mm in length. The forged pieces are charged in the vertical furnaceat 500° C. for an hour exposure at the charge temperature and subsequentheating for hardening up to 1050° C. is then carried out at a ratepermitted with heating capacity of the furnace. Duration of exposure at1050° C. is determined by the maximum section of a forged piece,particularly exposure lasts for 8 hours for a forged piece of 150 mm insection. Subsequent to the 8-hour exposure at 1050° C. oil hardening ofthe forged pieces is performed with their cooling in oil to 120° C. andsubsequent air cooling to 100° C. In at latest two hours the forgedpieces are charged in the furnace heated up to 450° C., followed byheating up to 655° C. at the rate of 40° C. per hour (first stage oftemper). Exposure duration at 655° C. is determined by the forged piecemaximum section for thermal treatment, particularly exposure at 655° C.lasts, on average, for 16 hours for forged pieces of 150 mm in section.The 16-hour exposure at 655° C. is followed by air cooling of the forgedpieces to room temperature. In at latest two hours subsequent to coolingto room temperature the forged pieces are charged into the furnace for asecond stage of temper which comprise the charging into the furnace at300° C., heating up to 635° C. at the rate of 55° C. per hour, exposurefor 12 hours at 635° C. followed by air cooling. Forged pieces of 510 mmin major diameter, 200 mm in minor diameter and about 4000 mm in lengthsubjected to said thermal treatment have required values of themechanical properties: a tensile strength of 93 to 106 kilograms persquare millimeter, a 0.2 percent yield strength of 77 to 94 kilogramsper square millimeter, an elongation of 14.2 to 20.0 percent, areduction in area of 46.5 to 61.5 percent, an impact strength determinedon specimens with semicircular notch of 10.9 to 16.6 kilogrammeters persquare centimeter, an impact strength determined on specimens withV-notch of 5.8 to 11.6 kilogram-meters per square millimeter.

As for forged pieces of 150 mm in section, it is desirable to subjectthem to at least three-stage temper. In particular, forged pieces of 540mm in section are subjected to thermal treatment as follows: charging ofthe forged pieces into the furnace at a temperature of 400° C. to heatthem for hardening, exposure for an hour at the charge temperature,heating up to 1050° C. at a rate enabled by heating capacity of thefurnace, exposure for 22 hours at 1050° C. The 22-hour exposure at 1050°C. is followed by oil hardening until the temperature on the forgedpiece surface is brought down to 120° C., then in at latest two hoursthe charging of the forged pieces into the furnace for a first stage oftempering is carried out, which first stage comprises the charging intothe furnace at a temperature of 300° C., exposure for three hours at thecharge temperature, heating up to a temperature of 650° C. at the rateof 50° C. per hour, exposure for 24 hours at 650° C., and air cooling ofthe forged pieces to 70° C. The second stage of tempering comprisescharging of the forged pieces cooled to 70° C. into the furnace at 300°C., exposure for two hours at the charge temperature, heating up to 640°C. at the rate of 50° C., exposure for 10 hours at 640° C., and aircooling of the forged pieces to room temperature. The third stage oftempering comprises charging of the forged pieces cooled to roomtemperature into the furnace at a temperature of 300° C., exposure fortwo hours at the charge temperature, heating up to a temperature 630° C.at the rate of 50° C. per hour, exposure for 10 hours at 630° C., andair cooling of the forged pieces to room temperature. Forged pieces of540 mm in section subjected to said thermal treatment have high valuesof the mechanical properties: a tensile strength of 94 to 99 kilogramsper square millimeter, a 0.2 percent yield strength of 77 to 81kilograms per square millimeter, an elongation of 15.5 to 21.5 percent,a reduction area of 41 to 64 percent, an impact strength determined onspecimens with semicircular notch of 9.3 to 17.7 kilogram-meters persquare centimeter, and an impact strength determined on specimens withV-notch of 7.0 to 10.9 kilogram-meters per square centimeter.

In the description of the various examples according to the presentinvention disclosed above, specific narrow terminology has been resortedto for the sake of clarity. It should be understood, however, that thepresent invention is no way limited to the terms so selected and thateach term covers all equivalent elements operating in a similar mannerand employed for solving similar problems.

Though the present invention has been described herein with reference topreferred embodiments thereof, it will be understood that minor chargesmay be made without departing from the spirit and scope of theinvention, as be readily understood by those skilled in the art.

We claim:
 1. A process for the manufacture of a corrosion-resistantweldable martensitic steel, residing in preparing a molten massessentially consisting of carbon 0.06 to 0.10 weight percent, chromium15.1 to 16.5 weight percent, nickel 3.5 to 4.45 weight percent, silicon0.10 to 0.60 weight percent, manganese 0.20 to 0.50 weight percent, atleast one element selected from the group consisting of niobium 0.25 to0.40 weight percent and zirconium 0.05 to 0.20 weight percent, at leastone element selected from the group consisting of yttrium 0.05 to 0.20weight percent, cerium 0.05 to 0.15 weight percent and lanthanum 0.05 to0.15 weight percent, phosphorus not exceeding 0.025 weight percent,sulfur not exceeding 0.02 weight percent, copper not exceeding 0.20weight percent, the remainder being substantially iron and unavoidableimpurities, pouring the molten mass into a mould and permitting it tosolidify therein followed by cooling the obtained ingot, said coolingstep being carried out in at least two stages, the first stage residingin cooling said ingot to a temperature laying in the martensitetransformation start-end interval but not lower than to 100° C. and inits immediate heating up to tempering temperatures ranging from 600° to650° C., each subsequent stage comprising cooling said ingot tomartensite transformation temperatures but by at least 50° C. lower thanthe cooling temperature of the previous stage, thus bringing with such amultistage cooling the temperature of the ingot down to a value belowthe temperature of the end of martensite transformation, followed byfinal tempering in the temperature range from 600° to 650° C. andsubsequent cooling to room temperature.
 2. A method for the manufactureof articles of a corrosion-resistant weldable martensitic steel,residing in preparing a molten mass essentially consisting of carbon0.06 to 0.10 weight percent, chromium 15.1 to 16.5 weight percent,nickel 3.5 to 4.45 weight percent, silicon 0.10 to 0.60 weight percent,manganese 0.20 to 0.50 weight percent, at least one element selectedfrom the group consisting of niobium 0.25 to 0.40 weight percent andzirconium 0.05 to 0.20 weight percent, at least one element selectedfrom the group consisting of yttrium 0.05 to 0.20 weight percent, cerium0.05 to 0.15 weight percent and lanthanum 0.05 to 0.15 weight percent,phosphorus not exceeding 0.025 weight percent, sulfur not exceeding 0.02weight percent, copper not exceeding 0.20 weight percent, the remainderbeing substantially iron and unavoidable impurities, pouring the moltenmass into a mould, permitting it to solidify therein, hot plasticworking of the obtained ingot and its subsequent cooling, said coolingstep being carried out in at least two stages, the first stage residingin cooling the article to a temperature laying in the martensitetransformation start-end interval but not lower than to 100° C. and inits immediate heating up to tempering temperatures ranging from 600° to650° C., each subsequent stage comprising cooling the article tomartensite transformation temperatures but by at least 50° C. lower thanthe cooling temperature of the previous stage, thus bringing with such amultistage cooling the temperature of the article down to a value belowthe temperature of the end of martensite transformation, followed byfinal tempering in the temperature range from 600° to 650° C. andsubsequent cooling to room temperature.